In the prior art, water-in-oil emulsion ink containing oil components has been mainly used for stencil duplicators and water based ink containing few oil components has been mainly used for ink-jet printers.
Unfortunately, in printers using such liquid ink, so-called curling occurs unlike in copiers for performing heat fusing with powder toner. In other words, when liquid ink is printed on a surface of a sheet during printing, the moisture of the ink quickly penetrates into the sheet from the surface and thus rolling, that is, curling rapidly occurs on the printed sheet.
During single-sided printing, a printed sheet is generally rolled from a surface on which liquid ink is printed to the opposite surface. Thus when the printed sheet is ejected to a paper receiving tray with the printed surface directed upward, the sheet is curled into a reversed U-shape from the front side, which is directed upward, to the back side.
During double-sided printing, the printed sheet is rolled from a surface having a large print rate to the other surface having a small print rate. Thus during the ejection of the printed sheet to the paper receiving tray, the printed sheet is curled downward into a reversed U-shape from the front side, which is directed upward, to the back side of the sheet when the front side has a higher surface print rate. The printed sheet is curled upward into a U-shape from the back side, which is directed downward, to the front side of the sheet when the back side has a higher print rate.
In a stencil duplicator of the prior art, printed sheets of paper are transported by a paper suction transport belt and are ejected from a paper ejecting device to a paper receiving tray according to a typical technique. The paper ejecting device includes so-called jumping boards for adjusting the shape of a printed and ejected sheet, which jumps from the paper ejecting device to the paper receiving tray, to a predetermined shape. With the jumping boards, the shape of the printed sheet is curved so as to raise the sheet ends provided in parallel with an axis disposed along a jumping direction.
Thus when the printed sheet is curled into a reversed U-shape (downward), the printed sheet is sucked by the paper suction transport belt and the surfaces of the printed sheet are shaped along a paper transport surface, so that curling of the sheet can be corrected. Moreover, with the jumping boards of the paper ejecting device, curling of the printed sheet is inverted and corrected and the shape of the printed and ejected sheet can be adjusted to a predetermined shape.
Although the center of the printed sheet is sucked by the paper suction transport belt in the paper ejecting device, the jumping boards have no air suckers. For this reason, even when curling of the sheet can be substantially corrected by sucking the center of the printed sheet by the paper suction transport belt and the surfaces of the sheet are shaped along the paper transport surface, sheets may be misaligned on the paper receiving tray depending on the kind of paper. In other words, parts around the sheet ends may rise from the paper transport surfaces of the jumping boards in a state in which the sheet ends are in contact with the paper transport surfaces of the jumping boards, so that curling may slightly remain around the sheet ends. Since curling cannot be positively corrected, the shape of the printed and ejected sheet cannot be adjusted to the predetermined shape, so that sheets are misaligned on the paper receiving tray.
When the printed sheet is curled into a U-shape (upward), the surfaces of the printed sheet cannot be shaped along the paper transport surfaces only by sucking the printed sheet by the paper suction transport belt.
Such a paper ejecting device is described in, for example, Japanese Patent Laid-Open No. 2007-246181 and Japanese Patent Laid-Open No. 2001-18512. In Japanese Patent Laid-Open No. 2007-246181, a paper transport unit for transporting sheets by air suction includes paper stiffness plates (corresponding to the jumping boards) for providing stiffness for a printed sheet which is ejected from a printer, and the paper transport surfaces of the paper stiffness plates have air suction holes for sucking the sheet on the transport surfaces.
In this invention, when the printed sheet is curled into a reversed U-shape (downward), a sheet surface opposed to the paper stiffness plate is sucked and thus both ends of the printed sheet are shaped along the paper transport surfaces of the paper stiffness plates, so that curling can be corrected.
In the foregoing configuration, however, a force applied to the printed sheet by air suction presses the surface of the printed sheet to the paper transport surfaces of the paper stiffness plates and thus friction between the surface of the printed sheet and the paper transport surfaces of the paper stiffness plates becomes a transport resistance. For this reason, depending on the kind of paper, for example, a thin sheet having low stiffness may be wrinkled.
Also in the foregoing configuration, when the printed sheet is curled into a U-shape (upward), a suction force of air suction does not effectively act on the surface of the printed sheet which is opposed to the paper stiffness plates, so that both ends of the printed sheet cannot be shaped along the paper transport surfaces of the paper stiffness plates.
The present invention has been devised to solve the problems. An object of the present invention is to provide a paper ejecting device which can positively correct curling, reduce friction on a paper transport surface to obtain smooth ejection, and adjust the shape of a printed and ejected sheet to a predetermined shape when the printed sheet is curled into a reversed U-shape (downward) and when the printed sheet is curled into a U-shape (upward).